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Because like, if i think in the state of the footage, having a 30 seconds CD on holy sanctuary... technically means that you can use only 3 heals every 30 seconds, if you use more then you will lose dps because you won't have enough holy sanctuary usages to benefit from it. So the way this skills currently works looks to me like it will actually limitate the numer of heals you want to use... While if they implement it the way i said before, then holy sanctuary won't have a usage limitation, so people will be able to heal as much as they want, they will just have to think about using holy sanctuary every 3 heals they use.
In that case, holy sanctuary would become more useful if it doesn't consume every sanctity stack, but only one every time : you could use 3 heals in order to prepare 3 sanctity so you could have 3 instant casts during an incoming movement heavy mechanic.
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Or actually (sorry for repost), we can mix both options : base damage gcd stays instant, and holy sanctuary keeps a cast time, but holy sanctuary doesn't have a CD and can be used only when you have at least 1 sanctity stack. That way, holy sanctuary would serve as a button "okay, you healed 3 times, now you are forced to stay put and cast holy sanctuary to gain the right to cast other heals without losing dps". That looks fine to me, you are not really limited in the number of heals you can use, and there is some kind of managing your cast time in order to get max dps.